Parity Nonconservation inF18and Meson-Exchange Contributions to the Axial Charge Operator

A simple relationship between the one-body and exchange-current contributions to the axial charge operator may allow a separation of these amplitudes in the $\beta$-decay transition ${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}(J^{\pi }I=0^{+}1)\to {}_{}{}^{18}{}_{}{}^{}\mathrm{F}(J^{\pi }I=0^{-}0)$. Since the exchange-current operator is related by isospin rotation to the pion-exchange component of the parity-nonconserving nucleon-nucleon potential, this suggests that the parity-nonconserving pion coupling can be extracted, with minimal dependence on nuclear models, from circular-polarization measurements for the analog $\gamma$ decay in ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$.